1. Field of the Invention
The present invention relates to an optical filter assembly and its method of manufacture.
2. Technical Background
Multiple-port, filtering and isolating packages are widely used in local and long distance optical telecommunication networks. These networks comprise various spectral shaping and isolating optical filter assemblies as parts of dense wavelength division multiplexing (DWDM) systems. The necessity to design reliable filters for such systems, which are subject to various thermal and mechanical loads during their 20 to 25 year lifetime, is of significant importance. A typical filter assembly comprises two (input and output) optical glass fibers inserted into a dual-capillary ferrule to produce a fiber-ferrule sub-assembly, a grated index (GRIN) lens, a spectral shaping (isolating) glass filters. The optical components of the assembly are embedded into an insulating glass tube, which in turn is mechanically protected by the metal housing (enclosure). In a typical 3-port package the above dual-fiber filter assembly is combined with the output collimating assembly leading to single optical fiber. The filter assemblies have been known, exhibit excessive insertion losses due to the coupling of the input fibers to the ferrule and the subsequent alignment of the collimator to the spectral shaping or isolating filter have been higher than desired, resulting in degraded overall performance of the system particularly during exposure to ambient operating conditions.
In prior art systems, input glass ferrules employ one of two major designs. Either a single capillary of elliptical cross section or separate circular capillaries have been used, each with relatively short (1.8 mm) fiber-receiving ends. With such input ferrules, the optical fiber is subjected to a S-bending over the short conical end portion which typically exceeds 50% of the fiber diameter (for a fiber having a 125 xcexcm diameter on a span of about 12 to 15 diameters in length. This excessive micro bending increases the insertion losses, Although the dual-capillary design reduces the lateral deflection of fiber interconnects compared to the elliptical single-capillary design, the short length of the cone end of such ferrules cannot reduce the micro-bending of the fiber and its inherent insertion loss. Fiber-ferrules subassemblies employing such ferrules are manufactured by the following steps of: fabricating the ferrules to hold the optical fibers (1); inserting the optical fibers stripped of their polymer coating into the respective ferrule capillaries (2); epoxy bonding them into the ferrule capillaries, including the conical end portions (3); grinding an 8xc2x0 facet of the fiber-ferrule (4); polishing the facet (5) and depositing on the polished surface an antireflection (AR) coating. Once finished, the fiber-ferrule is aligned and assembled with the GRIN or ball lens collimator, whose surface is coated with anti-reflection (AR) films, and then embedded into the insulating glass tube, which, in turn, is protected by a metal housing to provide structural integrity, robustness and thermal insulation to the assembly.
There are two different technical solutions used in the design of bonds securing the components of a filter assembly. A low compliance bond between thermally well matched fibers and the ferrule is an approach commonly used by a majority of manufacturers. The adhesives used are heat-curable epoxies with high Young""s modulus (E greater than 100,00 psi) and moderate to high thermal expansion coefficients (xcex1=40-60 10xe2x88x926xc2x0 C.xe2x88x921). A typical example would be 353 ND EPO-TEK epoxy adhesive. In addition, the bond thickness used is very small.
Silicon adhesives are used to bond thermally mismatched glass tubes with metal housings and glass filters with metal holders. In these joints, a high compliance design is used. The silicones, which can be cured between 20-150xc2x0 C. in the presence of moisture, are typically characterized by an extremely low Young""s modulus (E less than 500 psi) and high thermal expansion (xcex1=180-250 10xe2x88x926xc2x0 C.xe2x88x921). A typical example would be DC 577 silicone, which can be used to bond a metal filter holder to a GRIN lens.
Adhesive bonding with subsequent soldering or welding is required to encapsulate a filtering assembly into a three-port package or DWDM device. A precise alignment achieved during initial assembly of a filter prior to final packaging can be easily decreased due to the high temperature thermal cycles associated with soldering or welding during packaging of the component. Such prior art manufacturing processes and resulting components have several problems resulting from the fact that the optical components experience stresses due to the thermal contraction mismatch between the glass and metal materials; polymerization shrinkage in adhesive bonds; and structural constraints induced by bonding and final soldering during encapsulation. These stresses lead to displacements of optical components during bonding and soldering, resulting in 0.3-1 dB increase in the insertion loss.
Such a filter package enclosure, which is typically formed of six to eight concentric protective units, has micron transverse tolerances. Maintaining these tolerances requires precision machining, time-consuming alignment, and soldering with frequent rework. As a result of these limitations, the optical performance specifications are lowered and cost is increased. As an example, soldering typically includes several re-flow cycles. This induces local thermal stresses in the nearby adhesive bonds and leads to the degradation of the polymer adhesive, resulting in repositioning of optical components and a shift in the spectral filter performance. With such designs, soldering may also result in the contamination of optical components through direct contact with molten solder and/or flux.
Although both the collimating subassemblies and housings are cylinders, the alignment of commercially available optical components, which exhibit a random distribution of optical and structural characteristics, requires some lateral and angular repositioning of the subassemblies. This repositioning of the optical subassemblies is limited by the gap in the solder joint and the ratio of this gap to the length of the subassembly. The lateral and angular repositioning observed in some isolators can be as high as 0.05-0.3 mm and 0.5-1.5xc2x0, respectively. The soldering of non-capillary gaps meets well-known difficulties, such as high volume shrinkage of the solder, void formation, and contamination of optical components.
However, for many applications, it is desirable to obtain a high accuracy thermally compensated filtering or isolating three-port package that can be relatively inexpensive and reliable. Additionally, a package design should be adequate not only to mechanically protect the fragile optical components but also to compensate for and minimize the thermally induced shift in spectral performance. Thus, there exists a need for a process for manufacturing a filtering (or isolating) three-port package, which has a construction which is miniaturized, has a low insertion loss, is inexpensive to manufacture, and which results in a filter having reliable, long-term operation.
The present invention provides an improved optical filter assembly which provides a lower insertion loss, preferably below about 0.1 dB and allows the assembly of the optical components, such as an input ferrule, collimating lens and filter, utilizing bonding adhesives which allows the alignment of the individual components relative to one anther through an improved input ferrule and filter holder, which permits the utilization of UV and thermally curable adhesives and improved thermal curing to greatly reduce relevant internal stresses in the subassembly so formed.
Methods embodying the present invention include the steps of actively aligning a filter holder and filter to a collimator assembly including a GRIN lens mounted thereto, axially separating the filter holder and GRIN lens, in a movable fixture, placing a UV and thermally curable adhesive on the periphery of the GRIN lens, moving the GRIN lens into engagement with a filter holder having a filter mounted therein, adjusting the collimator assembly with respect to the filter holder while monitoring the input and output signals of the optical fibers coupled to the GRIN lens for insertion loss less than about 0.1 dB, and applying UV radiation through the filter end of the filter holder to initially cure the aligned subassembly. In a preferred embodiment of the invention, the subassembly is subsequently thermally cured through an accelerated dark cure sequence followed by thermal curing. In another embodiment of the invention, UV radiation is applied to the filter holder GRIN lens interface through one or more apertures formed in the side of the filter holder which overlap the GRIN lens. The light source may be dithered such that UV radiation uniformly covers the cylindrical interface between the holder and the outer surface of the GRIN lens. In a preferred embodiment of the invention, the filter and GRIN lens are prealigned prior to the application of adhesive by monitoring the input and output signals of the filter while adjusting the X-Y positioning for a maximum detected signal.
In a preferred method of manufacturing also, subsequent to the UV curing, the assembly is cured through a stress relaxation cycle at about 40-50xc2x0 C. for one to two hours followed by a thermal curing cycle of about 95xc2x0 C. for one to two hours.
In one embodiment of the invention, an input ferrule is employed with an input cone having an axial length greater than about 3 mm to reduce S-bending of input fiber, thereby minimizing resultant insertion losses. In another embodiment of the invention, a generally cylindrical filter holder having an annular seat formed in one end for receiving a filter and a lens-receiving aperture at an opposite end having an internal dimension which allows micro-tilting of the filer holder to provide an alignment at an angle of less than about 0.1xc2x0 to the axis of the GRIN lens. In one embodiment, the filter holder includes apertures, such as radially extending slots spaced around the periphery of the holder, to permit uniform UV curing of a UV and thermally curable bonding adhesive which, after adjusting for minimum insertion loss, secures the filter holder in the GRIN lens. An optical filter of a preferred embodiment of the present invention includes such an improved ferrule and/or a filter holder coupled in alignment with one another in a suitable housing.
The manufacturing method and filter assembly of the present invention, therefore, provides an improved performance filter utilizing a unique input ferrule, filter holder, and an assembly method for providing a low cost, highly reliable, and improved performance filter assembly, such as a three-port filter which can be used in an optical communication system.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.